378 



RODENTIA. 



pre-orbital ring, causes the latter to be much 

 elongated transversely. And towards its in- 



Fig. 258. 



Skull of the Ccelogynus. 



ner angle there is an excavation resembling 

 a long furrow or semi canal, which is really 

 the infra-orbital canal. The jugal is much 

 higher than it is broad ; the palatine extends 

 into the palate as far forward as the first molar 

 tooth : in the orbit it is almost hidden 

 by the projection of the maxillary ; ne- 

 vertheless, it is interposed between 

 that bone and the sphenoid, and at the 

 posterior extremity of the alveolar 

 arch. 



In the Guinea-pigs (AiicEnia, Fred. 

 Cuv. ; Cavia, Ilig. ; Musporcellus, Lin.), 

 the lachrymal is large ; but it does not 

 entirely form the upper [root of the 

 pre-orbital ring, and the maxillary is 

 not interrupted at this point. The pre- 

 orbital ring is much wider than it is high. 

 We may remark in this genus both the groove 

 of the paca and the fossa of the agouti; 

 but the latter is situated higher up, as in the 

 rats. The ascending branch of the maxillary 

 is long and narrow. The bones of the nose 

 are broader in front than behind- The jugal 

 only commences towards the middle of the 

 zygomatic arch ; the palatine, which superiorly 

 does not penetrate either into the orbit or into 

 the temporal region, extends in the palate 

 only as far forward as the interval between the 

 second and the third molar teeth. 



In the kerodons, a small point of the frontal 

 insinuates itself above, between the bones of 

 the nose and the intermaxillaries, the ascend- 

 ing branch of the latter being very long, and 

 extremely narrow at its origin, in the Bra- 

 zilian species. In this species, likewise, the 

 pre-orbital ring is oval, and much elongated 

 transversely, but formed entirely in the maxil- 

 lary bone as it is in the Guinea-pig ; whilst in 

 the kerodon of Patagonia the lachrymal forms 

 by itself nearly the whole vertical portion of 

 its arch, so that the lachrymal is necessarily 

 of very great size. Posteriorly, the maxillary 

 touches by its apex a long point derived from 

 the temporal external to the palatine ; the 

 latter, however, is enclosed between the sphe- 

 noid and the maxillary, and mounts up into 

 the floor of the orbit, when it is connected 

 with the lachrymal bone. In the palate it is 

 very deeply notched. 



In the capybara (Hydrochccrus, Er.xleben), 

 the jugal is still shorter than in the Guinea- 



pig. The lachrymal is largely developed at 

 the root of the vertical arch of the pre-orbital 

 ring, but does not assist in forming it. The 

 bones of the nose are very large and rectan- 

 gular. The ascending ramus of the inter- 

 maxillary, on the contrary, is extremely narrow, 

 and is only united by its point to a point de- 

 rived from the frontal. The inferior hori- 

 zontal arch of the ring is broad and flattened, 

 with a little fossa at its base, as in the kero- 

 dons ; the maxillary is connected behind with 

 the temporal, near the glenoid facet, external 

 to the palate bone ; but what distinguishes 

 capybara from them is, that this articulation 

 is much more extensive, and that we cannot 

 see, within, the long pterygoid apophysis and 

 that portion of the palatine alluded to above. 

 The external pterygoid alae are obliterated ; 

 the internal alae terminate by a rounded plate, 

 which is very far from reaching as far as the 



Fig. 259. 



Skull of the Capybara. 



tympanum. The palatine encroaches upon 



the palate as far forward as the third molar 



Fig. 260. 



Skull of the Chinchilla. 



a, b, c, portions of the temporal bone, which is here 

 very remarkable on account of the extraordinary 

 development of the tympanic cavity ; e, meatus 

 auditorius externus ; f, the occipital bone ; g, the 

 parietal ; h, h, the frontal ; i, zygomatic portion 

 of the temporal, which in this animal is quite 

 detached from the preceding portions ; k, malar 

 bone ; /, ossa nasi. 



